This invention relates to hydraulic throttle actuators such as those used for accelerating and decelerating an internal combustion engine which drives a pump in a hydraulic system. Particularly, the invention is directed to that type of throttle actuator which is operated responsive to pressure changes within the system resulting from the positioning of a selector valve and work demand.
Although the invention is primarily intended for use in conjunction with the prime mover of mobile equipment, it is adaptable for use with other hydraulic systems where the pump of the system is driven by an internal combustion engine. In many cases the hydraulic system is used only when the mobile equipment is in a stationary condition, the system itself being operated on an intermittent basis. The hydraulic systems generally are designed in such a manner that during inoperative periods, the valving in the system allows fluid, which is being pumped on a continuous basis, to pass through certain portions of the system and return back to the reservoir--thus fluid is in a constant state of movement whether the system is doing work or is in an idle condition. This type of system is referred to as an open-center hydraulic system. To those skilled in the art it will be apparent that certain rotary control valves, not shown, may be used, but for the purpose of the present disclosure reference will be made to a spool selector valve. In a system as herein described, when work is demanded of the system, the speed of the internal combustion engine is increased to cause the pump to apply fluid at higher pressures and flow rates. The most common means for controlling engine speeds utilizes fluid pressure from the hydraulic system itself, and is actuated only upon demand. Such means are normally called hydraulic throttle actuators. Although several hydraulically operated throttle actuators are presently available, they have operational disadvantages in that they are incapable of operating over the broad range of pressures encountered in such hydraulic systems and particularly are incapable of accurately sensing lower pressures.